Chore: Update `elixir-auth-github-demo` to `Phoenix 1.7`

[nelsonic]

I don't expect to this to take longer than T25M as and it should:

• not have any superfluous depdendencies/features, e.g. no mail, postgres, dashboard etc.
• should have the bare minimum code to make this work and nothing else.

[nelsonic]

Build failing on @dependabot PRs:
https://github.com/dwyl/elixir-auth-github-demo/actions/runs/6696640890/job/18194786113#step:8:123